How to solve this 8×8 logic puzzle

8×8 · severe · 24 steps

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This 8×8 board rates as severe (difficulty score 1349). Solving it from scratch takes 34 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, every remaining candidate in this region touches G7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, that one can't be the marker: at C1 it would force E2 (the last open cell in column E), then H3 (the last open cell in row 3), then G6 (the last open cell in region 7), and the chain continues, and row 7 would be left with no open cell for its marker.
Then, marking D1 would immediately leave column C with no open cell for its marker, so it can't be the marker there.
After that, that one can't be the marker: at F1 it would force E4 (the last open cell in region 3), and region 4 would be left with no open cell for its marker.
Now, marking G1 would force E4 (the last open cell in region 3), and then region 4 would be left with no open cell for its marker. So G1 can't be the marker there.
From there, if H1 were the marker, it would force G6 (the last open cell in region 7), then F8 (the last open cell in region 8), then E4 (the last open cell in region 3), and region 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
Following that, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
First, that one can't be the marker: at D2 it would force F3 (the last open cell in row 3), then H4 (the last open cell in region 3), then G6 (the last open cell in region 7), and region 8 would be left with no open cell for its marker.
Next, if F2 were the marker, it would force E4 (the last open cell in column E), and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
Then, that one can't be the marker: at G2 it would force E4 (the last open cell in region 3), and row 3 would be left with no open cell for its marker.
After that, marking H2 would force C3 (the last open cell in region 2), and then row 4 would be left with no open cell for its marker. So H2 can't be the marker there.
Now, every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
From there, every remaining candidate in this region touches D3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Following that, if E1 were the marker, it would force C2 (the last open cell in region 2), then H3 (the last open cell in row 3), then G6 (the last open cell in region 7), and the chain continues, and row 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
First, together, two regions have open cells only in columns A and B; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
Next, if C2 were the marker, it would force A1 (the last open cell in region 1), then B7 (the last open cell in region 5), then D6 (the last open cell in region 6), and the chain continues, and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
Then, place a marker at E2. Region has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region. Placing here also clears its 1 touching neighbour.
After that, place a marker at H3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
Now, place a marker at G6. Region has exactly one open cell left. Placing here clears the rest of row 6, column G, and its region. Placing here also clears its 1 touching neighbour.
From there, place a marker at F8. Region has exactly one open cell left. Placing here clears the rest of row 8, column F, and its region.
Following that, place a marker at D4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region. Placing here also clears its 1 touching neighbour.
First, place a marker at C7. Region has exactly one open cell left. Placing here clears the rest of row 7, column C, and its region.
Next, place a marker at A5. Region has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region.
Finally, place a marker at B1. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎